Nanocrystals and Nanoparticles Versus Molecular Fluorescent Labels as Reporters for Bioanalysis and the Life Sciences: A Critical Comparison - Advanced Fluorescence Reporters in Chemistry and Biology - page 11

Table 2 (continued)
Method Applications

Hybrid methods -Proteins can be directly

coupled to PEI amine
groups
-Silica can be easily
functionalized and then
bioconjugated

NH +

N

2 HN

NH +

N

2 HN

NH

NH 2

Bulky, partially charged ligands
(polyelectrolytes), e.g.,
polyethyleneimine (PEI) [ 40 ]

-

-

SiO 2

OH

-
-

OH

-
-

OH

-

-

-

Additional inorganic shells, e.g., silica
[ 41 , 42 ]

few to 10 nm. The size of the photoluminescence quantum yield of QDs is primarily
determined by the number of dangling bonds at the core particle's surface. Thus, the
modification of the surfaces of bare QDs is very important for the realization of high
fluorescence quantum yields. This can be achieved, e.g., by the deposition of a layer
of inorganic, chemically inert material or by organic ligands. Accordingly, in the
majority of cases, QDs present core-shell (e.g., CdSe core with a ZnS shell) or core-
only (e.g., CdTe) structures capped with specific organic or polymeric ligand
molecules. The most prominent materials for life science applications are currently
CdSe and CdTe. III/V group or ternary semiconductors such as InP, InGaP, CuInS 2,
and AgInS 2 - which lack cytotoxic cadmium ions - are possible alternatives that
have been synthesized and used recently [ 43 , 44 ]. At present, commercial products
are available for CdSe (Sigma-Aldrich, Invitrogen, Evident, Plasmachem), CdTe
(Plasmachem), and InP or InGaP (Evident).
Lanthanide (Ln) - or rare-earth-doped upconverting nanocrystals usually have
similar optical properties as their bulk counterparts [ 45 ]. Upconversion is char-
acterized by the successive absorption of two or more photons via intermediate